A 28-year-old male with glycogenosis type V associated with continuous hyperuricemia during mild daily activities is reported. An aerobic exercise test using a bicycle ergometer revealed that purine metabolites, i.e. ammonia, ino-sine, hypoxanthine and xanthine, were transiently increased by the exercise and that a subsequent increment in uric acid continued until the following day. The accelerated purine degradation by the muscle exercise was thus shown to be able to cause the overt hyperuricemia in a patient with glycogenosis type V. Therapeutic use of fructose for glycogenosis was disappointing due to fructose-induced hyperuricemia. A search for myogenic hyperuricemia is essential for therapeutic trials.